The investigators are conducting a double-blind, cross-over study (methylphenidate versus placebo) of normally intelligent, white, middle-class elementary school boys referred for academic and/or behavioral problems. Hyperactive, learn disabled (LD), and attention deficiting disordered (ADD) subjects are included. The study tests several interrelated hypotheses: (1) that the more hyperactive boys will have weaker or more excitable nervous systems as defined by Pavlovian investigators; (2) that strength of stability of the nervous system will related to response to psychostimulants; (3) that stability of the nervous system accounts, in part, for the phenomenon of augumentation-reduction as differences in cognitive styles. Nervous system strength is assessed before treatment via a contrast of press and release times to tones at four intensity levels. The project psychiatrist, blind to typing results, clinically adjusts to achieve best results. Preliminary analyses (N equals 40) show that those with excitable nervous systems, though of the same age, weigh significantly less than those with stable nervous systems and thus have been prescribed significantly higher dosages (mg./kg.) of methylphenidate. Stable types, however, improve as much on lower dosages as excitable types. Hyperactives have not been found to have more excitable nervous systems than LD and ADD subject.